In multiplayer, the quick trait's penalty is almost irrelevant. It's 1/20th of your unit's HP. Most of the time, this resolves to 1 HP. On units with a lot of HP, it's 2 HP. It doesn't really add up until they level up. Only in comparison to resilient units, especially ones with strong, when the quick unit lacks resilient, do quick units seem particularly fragile..

The one single trait I hate the most is intelligent. It's not like I wouldn't be happy about getting a clever unit, it's just that having fewer XP required to advance a level is such a marginal advantage most of the time that it simply isn't worth it, especially since the unit still has to kill some stuff before actually leveling up. Also, while it can at least sometimes be useful in multiplayer matches, I find it to be the absolute worst trait possible in any given campaign. You usually have lots of time to build a totally overpowered army anyway, so having leveled up units a few turns earlier is of next to no use, and they end up having weaker stats than units with other traits.

Also, while I generally like the trait "quick" a lot, I absolutely hate getting it on a unit that has exactly 6 movement points as a base value - no more, no less. It is useful on scouts to make them even "scoutier", it is very useful on 5-move-units to make them more maneuverable and to lessen the effect of heavy terrain on them, and it is useful on 4-move-units to "cure" their lameness. However, it is absolutely horrible on 6-move-units in my opinion, as having 7 moves still only allows you to go over 3 hexes with heavy terrain per turn, or 2 with very heavy terrain. That one additional move also doesn't really help if your unit would already have 6 moves as a base value, and with the HP reduction attached to it, I really find "quick" to be a penalty rather than a bonus on units with 6 base moves.

Thus, the non-plus-ultra of the absolute worst trait combo on any given unit but mages (with their massive XP requirement, intelligent is actually slightly useful to them, and quick benefits them unless you advance them to Silver Mages) is quick/intelligent, imo. Such units have fewer HP than normal, some units don't even need the extra move, and intelligent is much like a blank unless you get your unit close to leveling up in a multiplayer match. In a campaign, I tend to always dismiss units with this trait combo unless they got really close to leveling up.

Intelligent is very useful in multiplayer. Combined with the already-reduced experience requirements, intelligent units can level surprisingly fast, and become a threat of leveling more easily. Admittedly, it's more useful on some units than others. The thief, poacher, and augur end up needing only two kills and nothing else to level, and several others need two kills plus one or two experience points.

I just dislike quick. The only reason why I do is because it has a (Very unnoticable) negative effect. Woo! Strong gets +1 damage and +1 hp! On a mage! Wait? A Quick/Intelligent Mage? Ouch, even less hp. All the other traits are basically win-win situations, and even though quick is extremely useful, I still get bothered by it. Especially Quick/Intelligent. Hate that combo.

Funny enough, I like slow on my Goblins just because of the 5% hp increase, at least there's some benefit.

1. The 'strong' trait on Mages, Dark Adepts, Elvish Shamans and other units which are almost entirely ranged. Though it could be useful for the long-term advancement of a DA to L3.

2. The 'quick' Trait on 6-or-8-move units. Which is why I advance quick Spearmen into Halberdiers instead of Royal Guards, and Quick Horsemen into Grand Knights instead of Paladins.

3. Intelligence on units which max out at L2. The trait only makes an impression during the long stretch between L2 and L3, and in a few cases, between L3 and L4. Units which max at L2 do not apply to this.

And the best unit/trait combination I've seen? A dextrous/resilient Elvish Sylph.